Light-emitting diodes (LEDs) with low power consumption have recently been replacing conventional cold cathode fluorescent lamps (CCFLs) as light sources for the backlight device (also called “illumination device”) included in liquid crystal display devices. In backlight devices that use LEDs as the light source, a plurality of LEDs are arranged in a row with spaces therebetween on an end of a light guide plate. In liquid crystal display devices that include such a backlight device, images displayed on areas close to the LEDs become brighter, and images displayed on areas far from the LEDs become darker. As such, the attachment position of the LEDs leads to differences in brightness of the backlight radiated from the backlight device, resulting in unevenness in the brightness of the images displayed.
To reduce this kind of unevenness in the brightness of images, disclosed in Patent Document 1 is a light guide plate that has cut-out parts provided on a diffusion sheet attached to the surface of a light guide plate. FIG. 16 is a plan view showing the main parts of a liquid crystal display device 500 disclosed in Patent Document 1. As shown in FIG. 16, a plurality of LEDs 560 are affixed to an edge of a light guide plate 550 included in a liquid crystal display device 500. A diffusion sheet 570 and a prism sheet (an optical sheet) 575 are attached in this order to the surface of the light guide plate 550. Spheroid cut-out parts 570a are provided on the diffusion sheet 570 in positions corresponding to the respective LEDs 560 on a side thereof adjacent to the respective LED 560. Light emitted from the LEDs 560 is diffused by the cut-out parts 570a, and progresses and spreads uniformly through the light guide plate 550. This reduces unevenness in the brightness of the backlight.